Task #451: pin Notes header via an internal scroll container

Despite Tasks #448–#450 (sticky header + fixing .os-bg / html,body
overflow rules), the user kept reporting the bar disappears on
scroll. position: sticky is sensitive — the browser was picking
the wrong scroll container in this layout.

Robust fix: stop relying on document/body for scrolling on the
Notes page. Changed the page wrapper className in
artifacts/tx-os/src/pages/notes.tsx from
  min-h-screen os-bg flex flex-col
to
  h-screen os-bg flex flex-col overflow-y-auto

Now the Notes wrapper is a known scroll container that's exactly
one viewport tall. The sticky header inside it pins reliably to
the top edge of that container (== top of the viewport). Composer
flush-placement (Task #448), index.css overflow-x:clip on
html,body (Task #450), and other pages using .os-bg are all
untouched.

Single-line change, no other refactors.

artifacts/tx-os/src/pages/notes.tsx

@@ -304,7 +304,7 @@ export default function NotesPage() {
 
   return (
     <div
-      className="min-h-screen os-bg flex flex-col"
+      className="h-screen os-bg flex flex-col overflow-y-auto"
       dir={isRtl ? "rtl" : "ltr"}
       data-testid="notes-page"
     >